Chapter 4: Sex Determination & Sex-linked Characters

Chromosomal Sex Determination

Mechanism determining male and female phenotypes involves a complex system of gene expression and chromosomal behavior during gxametogenesis and fertilization. Males produce small, motile gametes known as sperm, while females produce larger, non-motile gametes referred to as ova; this difference in gamete size and functionality is a key aspect of sexual reproduction.

Sexual Reproduction

Formation of genetically distinct offspring occurs through the process of sexual reproduction, which begins with meiosis. This reductional division reduces the chromosome number, resulting in haploid gametes (sperm and eggs). These gametes unite during fertilization to form diploid zygotes, establishing the genetic makeup for the new organism.

Chromosomal Sex-Determining Systems

Chromosomes involved in sex determination vary between sexes in terms of morphology and quantity, creating diverse systems across species. Autosomes, which are non-sex chromosomes, include the X and Y chromosomes in mammals. During prophase I of meiosis, homologous chromosomes (including sex chromosomes) engage in synapsis, and sex chromosomes pair specifically in pseudoautosomal regions, allowing for proper segregation.

Types of Chromosomal Sex-Determining Systems

  1. X-Y System (Mammals)
    In this system, the male gamete (which can carry either an X or a Y chromosome) determines the sex of the offspring.

    • XX genotype results in a female (homogametic), while

    • XY genotype yields a male (heterogametic).

  2. X-O System (Some Insects)
    This system features one type of sex chromosome (X).

    • XX is female, while

    • XO denotes a male.

  3. Z-W System (Birds)
    In birds and some reptiles, the female gamete (which can carry either a Z or a W chromosome) determines the sex of the offspring.

    • ZW is female (heterogametic),

    • ZZ is male (homogametic).

  4. Haplo-Diploid System (Bees & Ants)
    In this system, the fertilization of eggs leads to females (diploid), while unfertilized eggs develop into males (haploid), illustrating a unique method of sex determination in these species.

Human Sex Determination

In humans, the X-Y system governs sex determination. The presence of the SRY gene on the Y chromosome triggers male phenotypical development by stimulating the formation of testes from bipotential gonads. Abnormalities in sex chromosome number, known as aneuploidy, can occur through nondisjunction during meiosis, often resulting in conditions that are less lethal than those that affect autosomal chromosomes.

Types of Aneuploidy

  1. Monosomy: A genetic condition characterized by the loss of one chromosome, represented as 2n-1.

  2. Trisomy: A genetic condition where an individual has an extra chromosome, shown as 2n+1.

Examples of Aneuploidy in Humans

  1. Triplo-X Female: A condition where individuals have an additional X chromosome (47, XXX). It occurs in approximately 1 in 1000 births and often presents with variable phenotypes, with some females experiencing developmental and reproductive challenges.

  2. Klinefelter Syndrome: Characterized by an extra X chromosome (47, XXY), this condition affects males, resulting in sterility, variable sexual development, and normal intelligence, with an incidence of about 1 in 1000 male births.

  3. Turner Syndrome: A condition characterized by the presence of only one X chromosome (45, XO). Affected individuals are often short in stature with a webbed neck and are typically sterile, occurring in around 1 in 3000 female births.

  4. XYY Condition: Males with an extra Y chromosome (47, XYY) may show associated behavioral issues; this condition has an incidence of about 1 in 1000 male births and is generally not associated with severe health issues.

Role of Sex Chromosomes

The X chromosome carries a plethora of essential genes that are critical for the development and function of both sexes. In the context of sex determination, the presence of the SRY gene induces male characteristics by initiating the development of testes. Females require two X chromosomes for proper ovarian function and fertility, whereas males only require one copy of the Y chromosome to express male-specific traits.

Human Y Chromosome Development

During embryonic development, gonads remain undifferentiated until approximately week 6 of gestation. At this stage, the SRY gene on the Y chromosome produces the Testis Determining Factor (TDF), which catalyzes the transformation of bipotential gonads into testes, leading to male differentiation.

Sex-linked Inheritance

Genes located on sex chromosomes exhibit distinct inheritance patterns compared to those located on autosomes. X-linked inheritance is characterized by traits that may be passed from carrier females to their male offspring. An example of this is the inheritance of eye color in Drosophila, demonstrating how sex-linked genes can express differently in males vs. females.

Thomas Hunt Morgan's Experiment

Significantly contributed to genetics by studying X-linked inheritance in fruit flies (Drosophila melanogaster), Morgan established that the trait of white eyes in these flies is recessive and can be transmitted through generations, further demonstrating the role of sex-linked genes in heredity.

Symbols for X-linked Genes

Standard notation includes a superscript to indicate alleles; for example, X+c signifies the normal allele, while Xc corresponds to the allele associated with red-green colorblindness.

Example Problem - Hemophilia

In the case of hemophilia, consider Sally who has a genotype of X+Xh (carrier) and Bob, who has X+Y. Their children's probabilities are:

  • Daughters have a 50% chance of being carriers (X+Xh) and 50% chance of being unaffected (X+X+).

  • Sons face a 50% chance of having hemophilia (XhY) and 50% chance of being healthy (X+Y).

Dosage Compensation

In female mammals, one of the two X chromosomes undergoes inactivation to equalize the gene dosage between males (who have one X chromosome) and females. This inactive X chromosome forms a structure known as a Barr body, leading to balanced expression of X-linked genes.

Effects of Aneuploidy in Humans

An example of aneuploidy that is frequently observed is Down Syndrome, which is characterized by Trisomy 21, where individuals possess an extra chromosome 21, leading to various developmental and cognitive challenges.

X-Chromosome Inactivation in Females

X-chromosome inactivation happens randomly during early embryonic development, ensuring that females express only one functional X chromosome, which is crucial for maintaining proper gene expression levels.

Examples of Mosaicism

In cases where females are heterozygous for X-linked traits, observed mosaics result from the random inactivation of one X chromosome, leading to different phenotypic expressions among cells. Anhidrotic ectodermal dysplasia is a particular condition that leads to patches of skin lacking sweat glands due to such mosaic expression.

Tortoiseshell Cats

This intriguing phenomenon in tortoiseshell cats stems from X-inactivation, resulting in a striking mosaic pattern of orange and black fur on their coats. The combination of these fur colors arises from the inactivation of one X chromosome in different patches, demonstrating how sex-linked genetic traits can manifest phenotypically in complex ways.

Homework Problems

For practice and reinforcement of concepts discussed, complete the following problems from the corresponding chapter: 1, 3, 4, 6, 9, 12, 15, 17, 20, 21, 23, 30, 35, 40, 41, 44, 47*